A119663 Triangular numbers with at most two distinct prime factors.

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 91, 136, 153, 171, 253, 325, 351, 496, 703, 1081, 1225, 1431, 1711, 1891, 2701, 3321, 3403, 4753, 5671, 7381, 8128, 12403, 13203, 13861, 15931, 18721, 25651, 29161, 29403, 31375, 32896, 34453, 38503, 49141

Offset: 1

Author

Greg Huber, Jul 28 2006

Keywords

• easy
• nonn

Comments

1 and 3 are the only terms with less than two prime factors.

Examples

			a(6) = 3 * 7, a(7) = 2^2 * 7, a(8) = 2^2 * 3^2.
		

Links

• Klaus Brockhaus, Table of n, a(n) for n=1..1000

Crossrefs

Cf. A000217, A068443, A005384.

Programs

Mathematica

Select[Accumulate[Range[0, 320]] ,PrimeNu[#]<3&] (* James C. McMahon, Sep 17 2024 *)

PARI

for(n=1, 320, k=binomial(n+1, 2); if(omega(k)<=2, print1(k, ", "))) \\ Klaus Brockhaus, Jul 30 2006

Extensions

More terms from Klaus Brockhaus, Jul 30 2006 and May 21 2008